Companion to the IV custom maps history. This focuses on the decline rather than the existence of the scene.

Three factors

Port quality. GTA IV's PC port had performance issues that lasted years. That made modding less appealing - mods often introduced more instability on top of the existing problems.

Audience timing. PC-modding culture was smaller in 2009 than it would be in 2015 when V launched.

V's pull. When GTA V launched on PC in 2015, most of the active modding community migrated. IV's scene shrunk by attrition.